A question about optical theorem

Optical theorem says that total cross section proportion to elastic scattering amplitude in the forward direction.Then if the target is absolutely reflective ''mirror''(in the case of quantum mechanics:scattering potential=infinity(r<a) and de Broglie wave leng limits to 0),in this case the scattering amplitude in forward direction is zero,but the total cross section is nonzero (because of reflection).So I do not understand the optical theorem.

In wave optics the forward scattering amplitude is never zero. I.e. even behind some metallic mirror (in the distance of several hundred meters) a bright spot will appear in the shadow zone.http://en.wikipedia.org/wiki/Arago_spot

But in case incident flux having diameter of the beam is very small comparision with diameter of the ''mirror'' and in classical limit(de Broglie wave leng tends to zero) and the momentum of particle is absolutely defined.So I think that having only reflection,but not having diffraction,then in this case there were not Arago spot?

In case the diameter of mirror large enough(the region of scattering potential),then we can consider the incident flux is plane wave despite of the diameter of incident beam is narrower than that of mirror(and in classical limit).In this case,with unitary property of S matrix we deduce the optical theorem.Then we are led to the transfer amplitude allways exists,despite of the scattering potential limits to infinity?